But there are 3 parties here: writer, reader and platform, for want of a better word.
It needs to reward them all to a point it is sustainable.
I don't frequent Substack much so maybe it's not apparent to me, but given it's business model promoting paid should be expected surely? Is that a dark pattern? Certainly feels it's low down the spectrum when compared with most online marketplaces.
On the other hand, taking revenue off paid subscriptions means they have two types of users, free and paying. Even inside paying customers they'll have classes, as not all subscriptions cost the same. This means that the company has an incentive to implement dark patterns that convert users from free to paid, or from lower tiers to higher tiers, even when that is not in the best interest of the writer.